Lord Weinstock, arguably Britain's greatest post-war industrialist, who transformed GEC - the General Electric Company - into the country's largest manufacturing group, died of cancer at his home in Wiltshire.

He led GEC - now known as Marconi - for 30 years, until his retirement in 1996.

From the family firm owned by his father-in-law, Weinstock built an (pounds) 11bn conglomerate which at its peak employed more than 240,000 workers.

Through a string of takeovers, including the Yarrow shipbuilders on the Clyde, GEC came to

dominate the United Kingdom's engineering, electronics, and telecommunications industries. Its interests ranged from torpedoes to fridges, and from petrol pumps to telephone exchanges.

But, since his retirement, he had witnessed the virtual destruction of the group caused by heavy investment in the telecom revolution. Lord Weinstock always remained silent on the subject of Marconi's troubles, but was privately understood to have been critical of the group's change in strategy under the direction of Lord Simpson.

Despite a reputation for caution which some believed led to a failure properly to exploit his company's potential, Weinstock's reputation as a great manager is assured. His business contemporary, Lord Hanson, said: ''He was without doubt the foremost manager of his time and one of whom one could say without reserve that he was No 1 - an icon. He had a complete grasp of his multitude of businesses. Sitting with him in his office he could open the drawer of his tightly packed filing cabinet and tell you the up-to-the-minute news of any single part of his empire. It will surprise no-one that he died of a broken heart when, after retirement, he realised he had made the one mistake of his brilliant career in turning over his company to a bunch of incompetents.''

Lord Prior, former Conservative cabinet minister and one-time chairman of GEC, said: ''He was a very outstanding post-war industrialist, perhaps the most outstanding. He had a brilliant mind and he did a great deal to help regenerate British industry.''

The influence of Weinstock and GEC was never greater than during the financial crises of the Callaghan government in the late seventies during which Professor William Gosling said: ''Without Sir Arnold and his company, Britain's balance of payments would be unmanageable.''

His high-profile reign at GEC was a far cry from humbler beginnings, the youngest of six children of Polish-Jewish parents who came to London in 1904.

Arnold Weinstock was born on July 29, 1924, and was an orphan by the age of 10.

He gained a degree in statistics at the London School of Economics and spent three years as a civil servant before going into business in finance and property development.

He joined what was to become GEC in 1954 at the invitation of his father-in-law. The company was then called Radio & Allied Industry and became GEC in 1960 following a takeover. Lord Weinstock took over as managing director in 1963.

In 1967, GEC swallowed Associated Electrical Industries and followed that with English Electric a year later in what was then the biggest merger in Britain.

GEC continued to expand under Lord Weinstock, with the acquisition of Yarrow Shipbuilders in 1974 and Avery in 1979. As a defence supplier, GEC's relations with government were paramount and it was Lord Weinstock's dexterity in this area which made him a hard act to follow.

Lord Weinstock was the darling of the Wilson governments and was hailed as an industrial hero. This view was not universally shared and the redoubtable Hugh Scanlon, leader of the AUEW engineering union once referred to Weinstock as ''Britain's largest unemployer''.

This referred to his essential management principle of rigorous financial control which generally involved closing loss-makers, cutting waste, and giving subsidiaries a free rein.

Weinstock likened industrial reorganisation to sculpting in marble. First you take a hammer and knock the material into shape, he said, then you go to work with a chisel.

''When things go wrong, I want to know why, in detail, and what is being done about it and who is OK and who is not,'' he once told managers in a letter complaining of ''astonishing complacency'' and ''duff budgets''.

His attention to detail was legendary and managers of factories he had never visited lived in dread of a late-night phone call quizzing them about a small matter in the latest accounts. He had a real horror of corporate profligacy, and even insisted that all lights be turned off at night at the electronics giant. However expensive his personal tastes, he regarded lavish corporate expenses, big company cars, and all the other time-honoured perks of big-business life with distaste.

Weinstock was knighted by Harold Wilson in 1970 for services to exports, and made a life peer 10 years later by Margaret Thatcher. He was to fall out with the Tory government over the privatisation programme and sat as a cross-bencher in the House of Lords.

Despite his ties to industry, Lord Weinstock had other passions - including music and horse racing.

He married his wife Netta, the daughter of the late Sir Michael Sobell, in 1949. It was with his father-in-law that Lord Weinstock indulged his love of racing, and they owned horses together from 1957. Over the next 30 years, many major races were won by a horse carrying the Weinstock and Sobell pale blue, yellow and white colours

In 1960 the pair purchased the 300-acre Ballymacoll Stud in County Meath, Ireland, along with about 130 horses for (pounds) 250,000.

In 1974 Lord Weinstock's son, Simon, became a partner in the stud with his father and took an active role in its management. He died at the age of 44 in May 1996.

He enjoyed a number of big race winners, the highlight of which was the success of Troy under Willie Carson in the 1979 Epsom Derby. Weinstock and Sobell's filly Sun Princess was another top-class performer taking the Epsom Oaks and the St Leger at Doncaster in 1983.

Last year Lord Weinstock's horse Golan, trained by Sir Michael Stoute, won the Sagitta 2000 Guineas at Newmarket.

An opera lover quite happy to travel to Milan to indulge his passion, Weinstock was a particular fan of anything conducted by Riccardo Muti and chose nothing but performances by Muti, which he himself had attended when he appeared on Desert Island Discs.

Lord Weinstock was a trustee of the British Museum and the Royal Philharmonic Society.

He is survived by his wife and daughter.

Lord Weinstock, industrialist; born July 29, 1924, died

July 23, 2002.